1. Field of the Invention
The present invention relates to a projection display device that displays an image by projecting a spatially intensity-modulated light.
2. Description of the Related Art
In a conventional projection display device employing a light valve (a reflection optical modulating element), an illumination optical system guides a light from a light source to the light valve, and the light valve performs a spatial intensity modulation of the light. Then, the spatially intensity-modulated light (image component light) is guided to a screen by a projection optical system, and an image is displayed on the screen.
A white light source, such as a high-pressure mercury lamp and a xenon lamp, or a multicolor light source, such as a light emitting diode (LED) and a laser, is used as a light source for the projection display device. Therefore, the illumination optical system is required to have a function of illuminating the light valve efficiently without impairing its image quality due to characteristics specific to each of these various light sources. In addition, in recent years, the projection display device further needs to have a high contrast for achieving an image representation with an impact.
In an area of a projection television, which is an application of the projection display device, a digital micro-mirror device (DMD®) is employed as the light valve to make a configuration of the optical system relatively simple. This light valve is formed by arranging a plurality of movable mirror elements corresponding to a plurality of pixels (for example, a few hundred thousand elements), respectively, in a plane. Furthermore, the light valve is configured such that an inclination angle of each of the mirror elements is changed in response to pixel information. In the projection display device including the light valve, the illumination optical system, the light valve, and the projection optical system are configured such that the image light passes through a projection lens toward the screen when the mirror element is inclined to an ON position, and the image light becomes out of an optical axis of the projection optical system when the mirror element is inclined to an OFF position.
In such a projection display device, a decrease in the contrast must be considered, which is due to a specific operation of the light valve. For example, a method of controlling the angle distribution of illumination light beams by arranging an aperture unit in the illumination optical system has been known as a technique for improving the contrast of the illumination optical system (Japanese Patent No. 4032658 (pages 4 to 10, and FIGS. 1, 8, and 9)).
In the case of the conventional technique described above, because the center axis of the illumination optical system is arranged inclined with respect to a normal line of the light valve, a regular reflection light reflected at the surface of the cover glass of the light valve travels toward a direction near the middle between the direction of the ON light beam and the direction of the OFF light beam. Therefore, an optical design is taken such that the OFF light beam and the regular reflection light are not incorporated into the projection optical system. However, in the conventional technique described above, there is a problem in that the contrast is decreased because of optical components such as a diffraction light caused by a micro-lattice structure of the light valve and a scattered light generated in the illumination optical system.